As is well known, dental casting alloys should provide a high degree of biocompatibility or inertness to the conditions in the mouth and good physical properties so that they will provide long lived usage. In addition, those alloys which are used to provide castings upon which porcelain coatings may be cast must provide good bonding characteristics to the porcelain coatings and other characteristics which are compatible with the porcelain coatings, such as similar coefficient of expansion, avoidance of discoloration of the porcelain, etc. Lastly, the alloy should process well during casting and be useful with commercially available porcelains.
Until recent years, gold alloys, usually gold/platinum alloys, have been preferred as dental casting materials because they have provided a highly desirable balance of properties. The commercially available dental porcelains have been formulated so as to be compatible therewith.
Recently, the escalating costs of gold and platinum have resulted in extensive efforts to find alternate alloy compositions which would afford acceptable properties at considerably lower cost. Base metal alloys have generally been found to suffer from one or more limitations such as lack of sufficient biocompatibility, lack of aesthetics, etc. As a result, over the last several years, there has been considerable activity in the development of palladium base alloys in an effort to make use of the nobility of lower cost palladium.
A number of palladium/silver alloys have been developed which simulate the appearance of platinum alloys and which provide a high degree of biocompatibility while still exhibiting useful casting and physical properties. However, the silver content has a tendency to oxidize at the porcelain firing temperature and to discolor the porcelains being fired thereon to provide the aesthetic coatings which are widely employed, particularly in anterior dental restorations. In applicant's copending application Ser. No. 174,759 filed Aug. 4, 1980, there is disclosed a palladium/silver alloy which has overcome much of the discoloration problem.
However, even the palladium/silver alloys of applicant's copending application require close control in the porcelain firing step and selection of the porcelains used in connection therewith. The generally available dental porcelains were formulated for use with high gold content alloys so as to exhibit a coefficient of thermal expansion which is typically 5-10 percent lower than the high gold content alloys. This results in placing the porcelain coating in compression after cooling from the firing temperature, thereby producing a stronger restoration when it is subjected to tensile loading.
The reduction or elimination of the gold content in some of the substitute alloys has caused difficulty in maintaining a sufficiently high thermal coefficient of expansion, which is desirably in the range of 0.66-0.72 percent at 500.degree. C. As indicated in applicant's above identified copending application, silver has been used to replace gold in an effort to provide a suitable coefficient of expansion but it readily oxidizes at the porcelain firing temperatures. The silver oxide then diffuses into the porcelain and undergoes ion exchange with sodium therein to produce a distinct uncontrolled discoloration of the porcelain which is aesthetically unacceptable.
Alloys for use as ceramo-metal restorations must also exhibit a desired balance of physical and mechanical properties The hardness must exceed 150 Vickers to withstand the abrasion of opposing teeth. To withstand the stress transmitted through the restoration, the alloy must have an offset yield strength at 0.1 percent of over 40,000 psi. Tensile elongation of at least 6 percent is required to allow the margins of the alloy to be adjusted after being placed in the patient's mouth.
Moreover, a dental casting alloy must be able to be soldered before the porcelain firing cycle. Since porcelain is fired at approximately 1000.degree. C., the alloy must possess a solidus of at least 1100.degree. C. to allow the solder to flow without starting to melt the casting. However, in order to allow the alloy to be cast with standard equipment found in dental laboratories, the liquidus temperature must not be greater than 1400.degree. C. Lastly, the alloy must also exhibit good bonding to dental porcelains which is represented by a minimum value for the maximum bending stress of 12,000 psi and a minimum value for the bending strength ratio (maximum bending stress/modulus of elasticity of the alloy) of 0.7.times.10.sup.-3.
It is an object of the present invention to provide a novel palladium dental alloy which exhibits a highly desirable balance of casting properties and physical properties, together with biocompatibility and freedom from discoloration of porcelain coatings which are fired thereon and which provide good bonding of the porcelain coatings fired thereon.
It is also an object to provide such an alloy which is relatively inexpensive when compared to gold and platinum alloys and which provides a balance of properties which is comparable thereto.
Another object is to provide such an alloy which may be cast and soldered relatively easily and which will provide excellent bonding to porcelain coatings fired thereon and avoid discoloration thereof.
A further object is to provide dental restorations comprising castings of such alloys and procelain coatings fired thereon, and wherein the porcelain coatings are essentially free from any discoloration and exhibit a high degree of bonding strength to the casting.